9. A person is rock climbing at an altitude of 800 m. He go up by 7 mph. and come down by 9 mph. what was his average speed?

A. 7.875 mph

B. 7.125 mph

C. 7 mph

D. 7.5 mph

Answer: A

Explanation:

If distance is same in both cases, average speed = $\dfrac{\text{total distance}}{\text{Total time}}$ = $\dfrac{{2xy}}{{x+y}}$

Given that, x = 7 mph, y = 9 mph

Average speed = $\dfrac{{2 \times 7 \times 9}}{{7 + 9}}$ = 7.875 mph

10. In the reading room of a library, there are 23 reading spots. Each reading spot consists of a round table with 9 chairs placed around it. There are some readers such that in each occupied reading spot there are different numbers of readers. If in all there are 36 readers, how many reading spots do not have even a single reader?

A. 8

B. none

C. 15

D. 16

Answer: C

Explanation:

As there are 23 spots so there are 23 tables and at each table there are 9 chairs.

The condition is:

Each table occupies different no. of people and as there are 36 people so these people can be arranged in each table: 1, 2, 3, 4, 5, 6, 7, 8.